1. Field of the Invention
The present invention is related to communications-satellite network control. More particularly, it is related to channel allocation in multiple-satellite communication systems.
2. Discussion of Related Art
Terrestrial cellular communication systems are well known. Multiple Satellite communication systems complement terrestrial cellular communication systems to augment traffic handling capacity and service areas where wire or cellular networks have not reached. Satellite systems came into existence in response to the need for efficient and economical mobile communications. In general, the satellites act as a transponder, or "bent pipe", receiving ground based transmissions from one location and beaming the repeated transmission back down to another location after amplification and frequency shifting, as is discussed in U.S. Pat. No. 5,448,623, incorporated herein by reference in its entirety.
The basic principles of ground-linked cellular network operations are similar to those of the satellite-linked cellular networks. In both types of networks, a broadcast link is shared by all user terminals for network administration purposes. Each base station in a ground-cellular network has its own distinctive signal link for this purpose, serving as a "beacon" that mobile user terminals detect as they move into that base station's coverage area. The analog cellular radio system assigns channels using a frequency reuse pattern. These channel allocations require base stations for each frequency. A multiple satellite system provides channels through use of electronics on-board the spacecraft.
Mobile users initially find and select a cellular channel by searching for the strongest administrative pilot signals sent by nearby gateways as beacons. When the cellular network connects a user's call, however, that call is assigned to its own individual circuit. Each satellite in a satellite-cellular network acts as a transponder between the network and its users, repeating whatever signal it receives and beaming the repeated signal back down to earth. Many satellite transponders pass the user's call along like a simple "bent pipe" would, providing only amplification and carrier-frequency translation.
Each satellite-cellular call is carried by a circuit made up of two-way links between a user and the satellite, and between the satellite and a gateway that links the satellite into ground-based communications networks as well as other satellite links. Cellular networks use three basic multiplexing techniques: Frequency-Division Multiple Access (FDMA); Code-Division Multiple Access (CDMA); and synchronous or asynchronous Time-Division Multiple Access (TDMA). The individual links will be pairs of carrier frequencies, pairs of CDMA tone or keycode references, or pairs of TDMA time-slice sequences or digitally-addressed packets.
In CDMA ground-cellular networks, only one fixed-bandwidth channel frequency need be assigned to each individual base station, since that channel is then multiplexed using spread-spectrum techniques that incorporate individual links into the channels carrier-frequency band using respective clock or keycode signals. For example the "128-Ary" Walsh-spreading codes can define 128 different spread-spectrum links per channel. Thus CDMA encoding can use its portion of the spectrum efficiently, but the frequencies it uses must all be contiguous.
In satellite-cellular networks, individual gateways characteristically support a much larger number of users than individual ground-cellular base stations. Therefore CDMA satellite networks require even more bandwidth than CDMA ground networks, making the use of non-contiguous frequencies in hybrid FDMA/CDMA networks' over-all bandwidth particularly advantageous for satellite-cellular networks. In hybrid FDMA/CDMA multiplexing, each FDMA channel is a separate CDMA encoding system.
Wide bands of contiguous carrier frequencies are available within the present world-wide spectrum allocation plan above 20 Ghz. However, lower, L-band and S-band frequencies between 1.61 GHz and 2.5 GHz, and C-band frequencies between 5 GHz and 7.075 GHz, are more advantageous for satellite-cellular operations. These frequencies are less sensitive to the attenuation and cross-polarization interference effects of rain and other atmospheric conditions encountered in the 1414 km low earth orbits used for satellite-cellular communications links. Thus the hybrid FDMA/CDMA coding is particularly advantageous as a means of increasing the link capacity of satellite-cellular networks at the preferred, lower end of the spectrum.
On the other hand, although CDMA encoding theoretically provides efficient use of the spectrum, the hybrid CDMA links within each FDMA channel are "soft". That is, although a given number of links can be encoded by a given spread spectrum technique, in theory, some lesser number will be usable in practice. It should be noted that the "spectral efficiency" of a network is the number of calls that can be linked, relative to the maximum possible number of links within the portion of the spectrum that is being used in a given area.
The operational CDMA link-capacity of the FDMA channels in these hybrid systems is affected by path gain, co-channel interference between FDMA channels and CDMA self-interference within one FDMA channel, among other things. Moreover, the usable link-capacity of each satellite-cellular channel is further reduced and complicated by the link diversity required by satellite motion and by satellite battery-power constraints.
These constraints introduce uncertainty into the allocation of CDMA links in response to user demand. For example, even for a simple three-gateway region having only two satellites in view, the calculation of actual CDMA link capacity for one FDMA channel consumes two weeks of computing time on a high performance computer system. Uncertainty about the usable capacity of an FDMA channel can result in under-utilization, and so, can impair the actual, attainable spectral efficiency of FDMA/CDMA networks.
Several satellite-cellular networks using digitally-addressed TDMA packets similar to those used in conventional fiber-optic networks are known in the prior art. One example is the Teledesic.SM. TDMA network. This type of multiplexing also provides theoretical spectral efficiency, without the wasteful complex capacity variability of CDMA networks.
However, packet-based TDMA satellite-cellular technology also requires large blocks of contiguous frequencies, blocks that are only available in the weather-sensitive gigahertz frequencies at the high end of the spectrum. Also, TDMA traffic is highly sensitive to time jitter, which necessitates the use of fixed tiling and complex, expensive, error prone "steerable-beam" satellite equipment, to protect the continuity of each link's time base.
Thus, a CDMA satellite network has distinct advantages over TDMA satellite networks, if the set of peculiarly troublesome CDMA link-allocation problems inherent in such network operations are solved.